U.S. Patent Application No. 62/035,272 filed 8 Aug. 2014 (Ref: FN-397P-US) discloses an optical system for simultaneously acquiring visible and infrared (IR) images of a scene including a face having an iris pattern, with a view to acquiring a near-infrared (NIR) image of the scene where the iris pattern extends sufficiently across an image sensor to enable iris based biometric identification to be performed.
In U.S. 62/035,272, the face was typically at a distance of between 200-250 mm and the focal length required to image such a face on an 8 megapixel (Mp) sensor was approximately 4.65 mm (with a demagnification of 200/4.65=43, one could use a sensor as small as 65 mm/43=1.5 mm diagonal−65 mm being an approximate distance between eyes). Such an optical system could be incorporated within the relatively shallow housings of modern image acquisition devices including smartphones, tablets and laptop computers so enabling iris-based identification of users to such devices.
However, where users do not wish to position their face as close as 200-250 mm to an image acquisition device for identification purposes and are only prepared to be imaged at for example, 400 mm, the required focal length for an imaging system such as disclosed in U.S. 62/035,272 would increase to the point where it could not be incorporated within a shallow housing.
For example, even with a dedicated IR optical system specifically designed to image eye regions within a face, a co-axial image sensor and lens including an entrance pupil optimized to image an object such as an eye region, 40 mm in size, at a distance of 400 mm at F/2.4 would require a focal length of between f=13.7 mm and f=7.1 mm depending on the type of optics employed. (For these two examples, the demagnification would be 400/13.7=29 and 400/7.1=56, leading to the image sizes of the eye region 40 mm/29=1.4 mm and 40 mm/56=0.7 mm respectively (40 mm being the approximate size of an eye).
In order to focus on objects at variable distances, either the lens or the image sensor would need to move relative to one another. In this case the total track length (TTL) of the imaging system could have to increase to greater than between 7.1 mm and 13.7 mm, so making implementation more unfeasible within shallow housing devices.